In addition to their well-known use for medical examination, X-rays have found increasing use for inspection purposes in manufacturing, e.g. for inspecting food products in containers for impurities that can be detected as having higher density than the substance under test and thus greater attenuation of applied X-rays. In a typical food product inspection station, a shielded head-end unit including an x-ray source and an x-ray sensor scans containers of food or beverages as they are moved sequentially through the head-end unit at a rate that can typically range up to 1000 containers per minute. While the containers are typically closely adjacent, there may be unpredictable periods of time during which the flow of product on a conveyor is interrupted, causing random gaps of substantial distance between adjacent containers.
Typically the x-ray source, sensor and conveyor driving mechanism are controlled from a control console which is located nearby in a separate enclosure and which may include a microprocessor along with electronic control and logic circuitry for implementing the inspection program.
Despite efforts to collimate the x-rays from the generator, i.e. direct them in parallel straight lines, confined to the product item under test and the sensor, the X-rays tend to diffuse and scatter whenever they collide with matter, and to thus escape through any openings in the shield structure; therefore, in the work environment, tight shielding is required to protect workers from harmful cumulative effects of exposure to extraneous x-ray radiation.
In the field of endeavor of the present invention where the product item is typically packaged food and beverage items such as bottled liquids moving along a conveyor, it is customary to surround the generator, product item under test, sensor and the associated portion of the conveyor with an enclosure constructed from high density X-ray shielding material; typically material of ultra high molecular weight is utilized to avoid excessive thickness requirements.
Of particular concern are the entry and exit openings that are required for product to flow through the test station: x-ray leakage through such openings may be minimized by providing shield tunnels and/or shield doors, however their shielding effectiveness depends somewhat on full loading and uniform close spacing of product containers within the test station to minimize radiation leakage as the containers move through on the conveyor. A gap in the loading of product moving along the conveyor could result in increased radiation leakage during the corresponding time period as the gap enters and/or exits the test station.
Since the health hazard effects of X-ray exposure are cumulative, the degree of risk is proportional to the product of exposure time duration and the level of radiation, so it is important to maximize the margin of safety by minimizing both the time duration and the level of the environmental radiation, and to take special measures to avoid even short periods of increased radiation levels.